Method of Double Patterning Lithography process Using Plurality of Mandrels for Integrated Circuit Applications

ABSTRACT

A method includes performing a double patterning process to form a first mandrel, a second mandrel, and a third mandrel, with the third mandrel being between the first mandrel and the second mandrel, and etching the third mandrel to cut the third mandrel into a fourth mandrel and a fifth mandrel, with an opening separating the fourth mandrel from the fifth mandrel. A spacer layer is formed on sidewalls of the first, the second, the fourth, and the fifth mandrels, wherein the opening is fully filled by the spacer layer. Horizontal portions of the spacer layer are removed, with vertical portions of the spacer layer remaining un-removed. A target layer is etched using the first, the second, the fourth, and the fifth mandrels and the vertical portions of the spacer layer as an etching mask, with trenches formed in the target layer. The trenches are filled with a filling material.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/087,334, entitled “Method of Double Patterning Lithography ProcessUsing Plurality of Mandrels for Integrated Circuit Applications,” filedon Nov. 22, 2013, which application is incorporated herein by reference.

BACKGROUND

Double patterning is a technology developed for lithography to enhancethe feature density. Typically, for forming features of integratedcircuits on wafers, the lithography technology is used, which involvesapplying a photo resist, and defining features on the photo resist. Thefeatures in the patterned photo resist are first defined in alithography mask, and are implemented either by the transparent portionsor by the opaque portions in the lithography mask. The features in thepatterned photo resist are then transferred to the manufacturedfeatures.

With the increasing down-scaling of integrated circuits, the opticalproximity effect posts an increasingly greater problem. When twoseparate features are too close to each other, the optical proximityeffect may cause the features to be shorted to each other. To solve sucha problem, double patterning technology is introduced. In the doublepatterning technology, the closely located features are separated to twophotolithography masks of a same double-patterning mask set, with bothmasks used to expose the same photo resist, or used to pattern the samehard mask. In each of the masks, the distances between features areincreased over the distances between features in the otherwise a singlemask, and hence the optical proximity effect is reduced, orsubstantially eliminated in the double patterning masks.

The double patterning, however, also suffers from drawbacks. Forexample, when two features have their lengthwise directions aligned to asame straight line, and the line ends of the features face each other,it is difficult to control the uniformity of the line end space due tothe proximity effect and overlay variation. The line widths of thefeatures are also difficult to control, especially when there are otherfeatures close to these two features.

The double patterning, however, also suffers from drawbacks. Forexample, when two features have their lengthwise directions aligned to asame straight line, and the line ends of the features face each other,it is difficult to control the uniformity of the line end space due tothe proximity effect and overlay variation. The line widths of thefeatures are also difficult to control, especially when there are otherfeatures close to these two features.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 15 are top views, perspective views, and cross-sectionalviews of intermediate stages in the manufacturing of features in atarget layer in accordance with some exemplary embodiments; and

FIGS. 16A through 16C illustrate a top view and cross-sectional views ofthe features formed in the target layer.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

Features with fine line spacing and the methods of forming the same areprovided in accordance with various exemplary embodiments. Theintermediate stages of forming the features are illustrated inaccordance with some exemplary embodiments. The variations of theembodiments are discussed. Throughout the various views and illustrativeembodiments, like reference numbers are used to designate like elements.

FIGS. 1 through 15 illustrate top views and cross-sectional views ofintermediate stages in the formation of features in a target layer inaccordance with some exemplary embodiments. Some of figures include atop view and a cross-sectional view of wafer 100 in the same figure,wherein the edges of the illustrated features in the top view may besubstantially aligned to the edges of the illustrated features in therespective cross-sectional view.

FIG. 1 illustrates wafer 100, which includes substrate 120 and theoverlying layers. Substrate 120 may be formed of a semiconductormaterial such as silicon, silicon germanium, or the like. In someembodiments, substrate 120 is a crystalline semiconductor substrate suchas a crystalline silicon substrate, a crystalline silicon carbonsubstrate, a crystalline silicon germanium substrate, a III-V compoundsemiconductor substrate, or the like. Active devices 122, which mayinclude transistors therein, are formed at a top surface of substrate120.

Dielectric layer 124 is formed over substrate 120. In some embodiments,dielectric layer 124 is an Inter-Metal Dielectric (IMD) or anInter-Layer Dielectric (ILD), which may be formed of a dielectricmaterial having a dielectric constant (k value) lower than 3.8, lowerthan about 3.0, or lower than about 2.5, for example. In someembodiments, conductive features 126, which may be metallic featuressuch as copper lines or tungsten plugs, are formed in dielectric layer124. Etch stop layer 26 is formed over dielectric layer 124. Etch stoplayer 26 may comprise a dielectric material such as silicon carbide,silicon nitride, or the like.

Dielectric layer 28 is further formed over etch stop layer 26.Dielectric layer 28 may be an IMD layer, which is formed of a dielectricmaterial having a dielectric constant (k value) lower than 3.8, lowerthan about 3.0, or lower than about 2.5, for example. In alternativeembodiments, dielectric layer 28 is a non-low-k dielectric layer havinga k value higher than 3.8.

In alternative embodiments, layer 28 is a semiconductor substrate,wherein the subsequent process steps may be used to form Shallow TrenchIsolation (STI) regions, for example. In these embodiments, there maynot be additional layers underlying layer 28. Throughout thedescription, layer 28 is also referred to as a target layer that is tobe etched, and in which a plurality of patterns is to be formed thereinin accordance with embodiments of the present disclosure.

Over low-k dielectric layer 28 resides dielectric hard mask 30, whichmay be formed of silicon oxide (such as tetraethylorthosilicate (TEOS)oxide), Nitrogen-Free Anti-Reflective Coating (NFARC, which is anoxide), silicon carbide, silicon oxynitride, or the like. The formationmethods include Plasma Enhance Chemical Vapor Deposition (PECVD),High-Density Plasma (HDP) deposition, or the like.

Metal hard mask 32 is formed over dielectric hard mask 30. In someembodiments, metal hard mask 32 comprises titanium nitride, titanium,tantalum nitride, tantalum, or the like. The formation methods includePhysical Vapor Deposition (PVD), Radio Frequency PVD (RFPVD), AtomicLayer Deposition (ALD), or the like.

Dielectric hard mask layer 34 is formed over metal hard mask 32.Dielectric hard mask layer 34 may be formed of a material selected fromthe same candidate material of dielectric hard mask layer 30, and may beformed using a method that is selected from the same group of candidatemethods for forming dielectric hard mask layer 30. Dielectric hard masks30 and 34 may be formed of the same material, or may comprise differentmaterials.

Mandrel layer 36 is formed over dielectric hard mask 32. In someembodiments, mandrel layer 36 is formed of amorphous silicon or anothermaterial that has a high etching selectivity with the underlyingdielectric hard mask 32.

Over mandrel layer 36 resides a tri-layer comprising under layer(sometimes referred to as a bottom layer) 38, middle layer 40 over underlayer 38, and upper layer 42 over middle layer 40. In some embodiments,under layer 38 and upper layer 42 are formed of photo resists, whichcomprise organic materials. Middle layer 40 may comprise an inorganicmaterial, which may be a nitride (such as silicon nitride), anoxynitride (such as silicon oxynitride), an oxide (such as siliconoxide), or the like. Middle layer 40 has a high etching selectivity withrelative to upper layer 42 and under layer 38, and hence upper layer 42is used as an etching mask for the patterning of middle layer 40, andmiddle layer 40 is used as an etching mask for the patterning of underlayer 38. After the application of upper layer 42, upper layer 42 ispatterned.

The patterned upper layer 42 includes openings 44 therein. As shown inthe top view (also in FIG. 1) of wafer 100, openings 44 may have stripshapes. In some embodiments, pitch P1 of openings 44 is about threetimes width W1 of openings 44. Throughout the description, the patternsof openings 44 are also referred to as line-A1 patterns.

FIG. 1 illustrates a first photo process. Next, a first etching processis performed to transfer the pattern in upper layer 42 to mandrel layer36, resulting in the structure shown in FIG. 2. During the etching step,upper layer 42, middle layer 40, and under layer 38 may be consumed. Ifany residue of upper layer 42, middle layer 40, and under layer 38 isleft after the patterning, the residue is also removed. The etching isanisotropic, so that openings 44 in mandrel layer 36 have the same sizesas their respective openings 44 in upper layer 42 (FIG. 1). Theremaining portions of mandrel layer 36 in FIG. 2 are referred to asintermediate mandrels, which include intermediate mandrels 36A and 36B.

FIGS. 3 and 4 illustrate a second photo and a second etching processperformed on mandrel layer 36. Referring to FIG. 3, bottom layer 48,middle layer 50, and upper layer 52 are formed. The materials of bottomlayer 48, middle layer 50, and upper layer 52 may be selected from thesame candidate materials of bottom layer 38, middle layer 40, and upperlayer 42 (FIG. 1), respectively. Upper layer 52 is patterned to formopenings 54 therein. Openings 54 may have width W1, which may besubstantially the same as the width W1 in FIG. 1, and pitch P1, which isthe same as the pitch P1 in FIG. 1. Throughout the description, thepatterns of openings 54 are also referred to as line-A2 patterns.Line-A1 patterns and Line-A2 patterns in combination defines thepatterns of mandrels 54 as in FIG. 4.

Next, as shown in FIG. 4, the second etching process is performed toextend openings 54 into mandrel layer 36. Bottom layer 48, middle layer50, and upper layer 52 are either consumed in the etching, or removed ifany is left after the etching. As a result, mandrel layer 36 includesboth openings 44 and 54, which may be distributed evenly in someembodiments. The remaining portions of mandrel layer 36 are referred toas mandrels 56. As shown in FIGS. 1 through 4, mandrels 56 are theresults of the line-A1 patterns (44) as shown in FIG. 1 and the line-A2patterns (54) shown in FIG. 3. Mandrels 56 may have width W2 equal toabout ⅓ width W1, and pitch P2 of mandrels 56 may be equal to about ahalf of pitch P1 (FIG. 1). Accordingly, the first photo and first etchand the second photo and the second etch, which are in combinationreferred to as a 2P2E double patterning process, results in W2 and pitchP2 to be smaller than the respective width W1 and pitch P1 (FIG. 1).Width W2 and pitch P2 may be lower than the photolithography limits.Mandrels 56 include mandrels 56A, 56B, 56C, and 56D in the illustratedexemplary embodiments. Mandrel 56A is a remaining portion ofintermediate mandrel 36A in FIG. 2, and mandrels 56B and 56C are theremaining portions of intermediate mandrel 36B in FIG. 2.

FIGS. 5 and 6 illustrate a cut lithography process, which is used to cutmandrel 56B into two portions. Referring to FIG. 5, bottom layer 58,middle layer 60, and upper layer 62 are formed. The material of bottomlayer 58, middle layer 60, and upper layer 62 may be selected from thesame candidate materials of bottom layer 38, middle layer 40, and upperlayer 42 (FIG. 1), respectively. Upper layer 62 is patterned to formopening 64 therein, wherein a middle portion of mandrel 56B isoverlapped by opening 64, while the end portions of mandrel 56B andother mandrels 56 are overlapped by upper layer 62.

Next, as shown in FIG. 6, an etching process is performed to extendopening 64 into mandrel 56B, so that mandrel 56B is cut into twoportions 56B1 and 56B2. Bottom layer 58, middle layer 60, and upperlayer 62 are either consumed in the etching, or removed if any is leftafter the etching. As shown in the top view in FIG. 6, the resultopening in mandrel layer 36 includes an I-shaped opening, which includesopenings 44 and 54, and opening 64 interconnecting openings 44 and 54.

Referring to FIG. 7, spacer layer 76 is blanket formed over the wafer100 in FIG. 6. The material of spacer layer 76 may be selected from thesame group of candidate materials for forming metal hard mask 32(FIG. 1) or other materials such as dielectric materials that aredifferent from the material of mandrels 56. Furthermore, the material ofspacer layer 76 is selected to have a high etching selectivity withdielectric hard mask layer 34 (FIG. 1). For example, the material ofspacer layer 76 may be selected from AlO, AlN, AlON, TaN, TiN, TiO, Si,SiO, SiN, metals, and metal alloys.

As also shown in FIG. 7, spacer layer 76 is formed as a conformal layer,with the thickness T1 of its horizontal portions and the thickness T2 ofits vertical portions close to each other, for example, with adifference between T1 and T2 smaller than about 20 percent of thicknessT1. The top view in FIG. 7 is obtained from the horizontal planecontaining C-C in the cross-sectional view.

An anisotropic etching is then performed to remove the horizontalportions of spacer layer 76, while the vertical portions of spacer layer76 remain, and are referred to as spacers 80 hereinafter. The resultstructure is shown in FIG. 8.

When spacer layer 76 (FIG. 7) is formed, thickness T2 of spacer layer 76is selected to be equal to or greater than a half of width W3 (FIG. 5)of opening 64. As a result, as shown in FIG. 8, the sidewall (vertical)portions of spacer layer 76, which sidewall portions are in opening 64and on opposite sidewalls of mandrels 56B1 and 56B2, merge with eachother, as shown in FIG. 7. After the etching as in FIG. 8 is performed,spacers 80 remain to fill an entirety of opening 64. On the other hand,as shown in FIG. 8, openings 44 and 54 have center portions 44′/54′remaining not filled by spacers 80. Dielectric hard mask 34 is exposedthrough openings 44′/54′.

FIGS. 9A and 9B illustrate a perspective view and a top view,respectively, of portion 82 in FIG. 8. As shown in FIG. 9B, opening 44′includes protruding portion 44″ connected to major portion of opening44′, which major portion is rectangular. Opening 54′ includes protrudingportion 54″ connected to major portion of opening 54′, which majorportion is rectangular.

In FIGS. 10 and 11, some undesirable mandrels and mandrel portions, suchas mandrels 56B (including 56B1 and 56B2, refer to the top view of FIG.10) and 56C in FIG. 10, are removed. The process steps are discussed asfollows. Referring to FIG. 10, bottom layer 68, middle layer 70, andupper layer 72 are formed. The material of bottom layer 68, middle layer70, and upper layer 72 may be selected from the same candidate materialsof bottom layer 38, middle layer 40, and upper layer 42 (FIG. 1),respectively. Openings 74 (including 74A and 74B) are formed in upperlayer 72. Mandrel 56B and 56C is overlapped by opening 74A, while aportion of mandrel 56D is overlapped by opening 74C.

Next, an etching process is performed to remove mandrel portions 56B1and 56B2 and 56C. The etching is selective so that spacers 80 are notattached, while the exposed mandrels 56 are removed. For example, a partof mandrel 56D is removed, while a portion of mandrel 56D may remain.The resulting openings are shown in FIG. 11, and are referred to asopenings 83.

As shown in FIG. 11, openings 83A are formed by removing mandrelportions 56B1 and 56B2 (FIG. 10). Throughout the description, thepatterns of openings 83A are referred to as line-B patterns, which havelengthwise directions aligned into a straight line, with the ends of theline-B patterns closely located from each other.

Referring to FIG. 12, mandrels 56 and spacers 80 are in combination usedas an etching mask to etch the underlying dielectric hard mask 34, sothat openings 44′, 54′, and 83 extend into dielectric hard mask 34. Inthe respective process, mandrels 56 and spacers 80 may or may not befully consumed.

Next, dielectric hard mask 34 is used as an etching mask to etch metalhard mask 32. Mandrels 56 and spacers 80 may be consumed in thisprocess. The resulting structure is shown in FIG. 13. In FIG. 14, thepatterned hard mask 32 is used as an etching mask to etch the underlyingdielectric hard mask 30, low-k dielectric layer 28, and etch stop layer26, so that trenches 84 are formed. Additional process steps are alsoperformed to define and etch low-k dielectric layer 28 to form viaopenings 86 underlying trenches 84. Although trenches 84 and viaopenings 86 have the same widths in the illustrated plane, in a verticalplane perpendicular to the illustrated plane, via openings 86 havesmaller widths than trenches 84.

FIG. 15 illustrates the filling of trenches 84 and via openings 86 (FIG.14) to form metal lines 88 and vias 90, respectively. The formation mayinclude a dual damascene process, wherein a conductive barrier layersuch as titanium nitride, titanium, tantalum nitride, tantalum, or thelike is formed on the sidewalls and the bottoms of trenches 84 and viaopenings 86. The remaining portions of trenches 84 and via openings 86are then filled with a filling metal such as copper or copper alloy. AChemical Mechanical Polish (CMP) is then performed to remove excessportions of the barrier layer and the filling metal, forming metal lines88 and vias 90 as shown in FIG. 15. Metal lines 88 and vias 90 areelectrically connected to the underlying conductive features 126.

In alternative embodiments, target layer 28 is formed of a semiconductormaterial. Accordingly, the process step shown in FIGS. 1 through 14 maybe used to form trenches in target layer 28, and filling the trencheswith a dielectric material to form STI regions.

FIG. 16A illustrates a top view of metal lines 88 formed in low-kdielectric layer 28. As shown in FIG. 16A, metal lines 88 include 88A,88B, 88C, and 88D. Metal lines 88A and 88B are parallel to each other,and are closely located. Metal lines 88A and 88B are formed fromopenings 44′ and 54′ (FIG. 11). Metal lines 88C and 88D are locatedbetween metal lines 88A and 88B. Metal lines 88C and 88D are formed fromopenings 83A (FIG. 11). The lengthwise directions (and the lengthwisecenter lines) of metal lines 88C and 88D are aligned to the samestraight line 21. In accordance with some embodiments, line end space Sibetween metal lines 88C and 88D is between about 5 nm and about 100 nm.It is appreciated, however, that the values recited throughout thedescription are merely examples, and may be changed to different values.

As shown in FIG. 16A, metal line 88A includes main portion 88A1, whichis rectangular, and tip 88A2 protruding beyond edge 88A3 and toward thespace between metal lines 88C and 88D. Similarly, metal line 88Bincludes main portion 88B1, which is rectangular, and tip 88B2protruding beyond edge 88B3 and toward the space between metal lines 88Cand 88D. The tip portions are formed due to the formation of spacers 80,as shown in FIG. 9B, wherein openings 44′ and 55′ have tip portions.

FIGS. 16B and 16C are cross-sectional views of the structure shown inFIG. 16A, wherein the cross-sectional views are obtained from thevertical plane containing lines A-A and B-B, respectively, in FIG. 16A.

The embodiments of the present disclosure have some advantageousfeatures. By adopting the 2P2E process to form metal strips, the widthsand spacing of the resulting features are smaller than the limit oflithography processes. Further combining the 2P2E process with the linecutting process, metal lines 88C and 88D (FIG. 18) are formedsimultaneously, and do not have double patterning overlay issues. Thevariation in the line end space is thus minimized. Furthermore, the lineend space of features 88C and 88D may be very small since is equal tothe minimum size defined by the respective lithography technology.

In accordance with some embodiments, a method includes performing adouble patterning process to form a first mandrel, a second mandrel, anda third mandrel parallel to each other, with the third mandrel beingbetween the first mandrel and the second mandrel, and etching the thirdmandrel to cut the third mandrel into a fourth mandrel and a fifthmandrel, with an opening separating the fourth mandrel from the fifthmandrel. A spacer layer is formed on sidewalls of the first, the second,the fourth, and the fifth mandrels, wherein the opening is fully filledby the spacer layer. Horizontal portions of the spacer layer areremoved, with vertical portions of the spacer layer remainingun-removed. A target layer is etched using the first, the second, thefourth, and the fifth mandrels and the vertical portions of the spacerlayer as an etching mask, with trenches formed in the target layer. Thetrenches are filled with a filling material.

In accordance with other embodiments, a method includes forming amandrel layer over a target layer, performing a first lithography andetching process to pattern the mandrel layer, performing a secondlithography-and-etching process and a cut-etch process to patternremaining portions of the mandrel layer to form a first opening in themandrel layer. The first opening has an I-shape and includes twoparallel portions, wherein the two parallel portions are formed by thefirst lithography-and-etching process and the secondlithography-and-etching process. The first opening further includes aconnecting portion interconnecting the two parallel portions. Spacersare formed on sidewalls of the first opening, wherein the spacers fillan entirety of the connecting portion, and wherein a center portion ofeach of the two parallel portions is unfilled by the spacers. Themandrel layer is etched to remove a portion of the mandrel layer and toform a second opening, wherein the second opening is between the twoparallel portions of the first opening. The second opening is spacedapart from the two parallel portions of the first opening by thespacers. The method further includes extending the first opening and thesecond opening into the target layer.

In accordance with yet other embodiments, a method includes forming amandrel layer over a target layer, and performing a firstlithography-and-etching process to pattern the mandrel layer, whereinremaining portions of the mandrel layer include a first intermediatemandrel and a second intermediate mandrel are formed. The methodincludes performing a second lithography-and-etching process, wherein asize of the first intermediate mandrel is reduced to form a firstmandrel, and the second intermediate mandrel is cut into a secondmandrel and a third mandrel, wherein the first mandrel, the secondmandrel, and the third mandrel are parallel to each other, with thesecond mandrel being between the first mandrel and the third mandrel.The method further includes etching the second mandrel to cut the secondmandrel into a fourth mandrel and a fifth mandrel, with an openingseparating the fourth mandrel from the fifth mandrel. A spacer layer isformed on sidewalls of the first mandrel, the second mandrel, the fourthmandrel, and the fifth mandrel, wherein the opening is fully filled bythe spacer layer. Horizontal portions of the spacers are removed, withvertical portions of the spacer layer remaining un-removed.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: forming a mandrel encirclinga first opening and a second opening, wherein the first opening and thesecond opening are separated from each other by a first mandrel strip;etching a middle portion of the first mandrel strip to form a thirdopening connecting the first opening to the second opening, wherein thefirst mandrel strip comprises a first portion and a second portionremaining on opposite sides of the third opening; forming a blanketspacer layer over the mandrel; etching horizontal portions of theblanket spacer layer to form spacers, wherein the third opening is fullyfilled by the spacers, and the first opening and the second opening arenarrowed by the spacers; etching the first portion and the secondportion of the first mandrel strip to form a fifth opening and a sixthopening encircled by the mandrel and the spacers; using the mandrel andthe spacers as an etching mask to etch a target layer, with trenchesformed in the target layer; and filling the trenches with a fillingmaterial.
 2. The method of claim 1, wherein the forming the mandrelcomprises: forming a blanket mandrel layer over the target layer;performing a first etching to remove some portions of the blanketmandrel layer; and performing a second etching on remaining portions ofthe blanket mandrel layer to form the mandrel.
 3. The method of claim 1further comprising, removing the mandrel and the spacers.
 4. The methodof claim 1 further comprising, after the etching the horizontal portionsof the blanket spacer layer and before the target layer is etched,removing a second mandrel strip parallel to the first mandrel strip. 5.The method of claim 1 further comprising, when the fifth opening and thesixth opening are formed, simultaneously etching a portion of themandrel to form a third mandrel strip parallel to the first mandrelstrip.
 6. The method of claim 1, wherein the target layer comprises adielectric material, and wherein the filling the trenches comprisesfilling a metallic material.
 7. The method of claim 1, wherein themandrel comprise amorphous silicon.
 8. A method comprising: forming amandrel layer over a target layer; patterning the mandrel layer to forma first opening in the mandrel layer, wherein the first opening has anI-shape and comprises: two parallel portions; and a connecting portioninterconnecting the two parallel portions; forming spacers on sidewallsof the first opening, wherein the spacers fill an entirety of theconnecting portion, and the two parallel portions are narrowed by thespacers; etching the mandrel layer to remove portions of the mandrellayer on opposite ends of the connecting portion to form a secondopening and a third opening, wherein the second opening and the thirdopening are between the two parallel portions of the first opening; andextending the two parallel portions of the first opening, the secondopening, and the third opening into the target layer.
 9. The method ofclaim 8 further comprising: etching a hard mask underlying the mandrellayer to extend the second opening, the third opening, and remainingportions of the first opening into the hard mask, wherein the targetlayer is etched using the hard mask as an etching mask.
 10. The methodof claim 8 further comprising, after the second opening, the thirdopening, and remaining portions of the first opening are extended intothe target layer, filling the first opening and the second opening witha material different from the target layer.
 11. The method of claim 10further comprising removing the spacers.
 12. The method of claim 8,wherein the forming the spacers comprises: forming a spacer layer havinga thickness equal to or greater than a half of a width of the connectingportion of the first opening; and performing an anisotropic etching onthe spacer layer, wherein portions of the spacer layer remaining afterthe anisotropic etching are the spacers.
 13. The method of claim 8,wherein in the extending, the mandrel layer and the spacers incombination are used as an etching mask for etching the target layer.14. The method of claim 8, wherein the patterning the mandrel layer toform the first opening comprises: performing a firstlithography-and-etching process to form a first one of the two parallelportions; and performing a second lithography-and-etching process toform a second one of the two parallel portions.
 15. A method comprising:forming a mandrel layer over a target layer; performinglithography-and-etching processes to pattern the mandrel layer, withremaining portions of the mandrel layer comprising a first mandrel, asecond mandrel, and a third mandrel parallel to each other, with thesecond mandrel being between the first mandrel and the third mandrel;etching the second mandrel to cut the second mandrel into a fourthmandrel and a fifth mandrel, with an opening separating the fourthmandrel from the fifth mandrel; forming a blanket spacer layer over thefirst mandrel, the second mandrel, the fourth mandrel, and the fifthmandrel; and performing an anisotropic etching on the blanket spacerlayer, wherein the opening is fully filled by a remaining portion of theblanket spacer layer.
 16. The method of claim 15 further comprising:using the first mandrel, the second mandrel, the fourth mandrel, and thefifth mandrel and the remaining portions of the blanket spacer layer asan etching mask to etch the target layer, with trenches formed in thetarget layer; and filling the trenches with a filling material.
 17. Themethod of claim 16, wherein the target layer comprises a dielectricmaterial, and the filling the trenches comprises filling a metallicmaterial to form metal lines and vias.
 18. The method of claim 15,wherein the first mandrel, the second mandrel, and the third mandrelcomprise amorphous silicon.
 19. The method of claim 15 furthercomprising, after the anisotropic etching and before the target layer isetched, removing a sixth mandrel parallel to the first mandrel.
 20. Themethod of claim 15 further comprising, after the anisotropic etching andbefore a layer underlying the blanket spacer layer is etched, removing afirst portion of a sixth mandrel, with a second portion of the sixthmandrel remaining.